Conformal graphene cage encapsulated battery electrode materials and methods of forming thereof

ABSTRACT

A conformal graphene-encapsulated battery electrode material is formed by: (1) coating a battery electrode material with a metal catalyst to form a metal catalyst-coated battery electrode material; (2) growing graphene on the metal catalyst-coated battery electrode material to form a graphene cage encapsulating the metal catalyst-coated battery electrode material; and (3) at least partially removing the metal catalyst to form a void inside the graphene cage.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/175,148, filed on Jun. 12, 2015, the disclosure of which isincorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under contractDE-AC02-76SF00515 awarded by the Department of Energy. The Governmenthas certain rights in the invention.

TECHNICAL FIELD

This disclosure generally relates to conformal graphene cageencapsulated materials and, more particularly, conformal graphene cageencapsulated battery electrode materials.

BACKGROUND

Rechargeable lithium-based batteries with high energy density have beenintensely investigated to meet the ever-growing demands of portableelectronics and electrical vehicles. A variety of emerging anode andcathode materials have attracted much attention, including silicon (Si),tin (Sn), and lithium (Li) metal for anodes, and sulfur (S) and oxygen(O₂) for cathodes. Among these materials, Si is an attractive anodematerial for next-generation lithium-ion batteries (LIBs), havinggreater than about ten times the theoretical capacity of commercialgraphite anodes. However, challenges arise due to the large volumeexpansion of Si (about 300%) during battery operation, causing (1)mechanical fracture, (2) loss of inter-particle electrical contact, and(3) repeated chemical side reactions with an electrolyte.

It is against this background that a need arose to develop theembodiments described in this disclosure.

SUMMARY OF DISCLOSURE

Some aspects of this disclosure relate to a method of forming aconformal graphene-encapsulated material. In some embodiments, themethod includes: (1) coating a battery electrode material with a metalcatalyst to form a metal catalyst-coated battery electrode material; (2)growing graphene on the metal catalyst-coated battery electrode materialto form a graphene cage encapsulating the metal catalyst-coated batteryelectrode material; and (3) at least partially removing the metalcatalyst to form a void inside the graphene cage.

Additional aspects of this disclosure relate to a conformalgraphene-encapsulated material. In some embodiments, thegraphene-encapsulated material includes: (1) a graphene cage defining aninternal volume; and (2) a battery electrode material disposed withinthe internal volume, where the battery electrode material occupies lessthan 100% of the internal volume to define a void.

Other aspects and embodiments of this disclosure are also contemplated.The foregoing summary and the following detailed description are notmeant to restrict this disclosure to any particular embodiment but aremerely meant to describe some embodiments of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of some embodimentsof this disclosure, reference should be made to the following detaileddescription taken in conjunction with the accompanying drawings.

FIG. 1: Design and structure of graphene cage encapsulation. a, Simicroparticles fracture and lose electrical contact during repeatedbattery cycling. Freshly exposed surfaces of Si continually react withan electrolyte, resulting in a thick and ionically insulating solidelectrolyte interface (SEI) layer. The destruction of electrical andionic pathways leads to severe battery performance decay. b, Thegraphene cage imparts its mechanical strength, electrical conductivity,and chemical stability to microscale Si, addressing its majorchallenges. The mechanically flexible graphene cage confines theexpansion and fracture of Si microparticles, while remaining intact toelectrically connect the ruptured particles. Conductive additives areunnecessary even for thick electrodes due to the graphene cage's highelectrical conductivity. Efficient SEI formation on the graphite-likesurface of the graphene cage reduces irreversible Li ion loss, resultingin high initial and later-cycle Coulombic efficiency and fastcharge-transfer kinetics at the surface. These features allow stablecycling of Si microparticles.

FIG. 2: Synthesis and characterization of graphene cage structure. a,Schematic of dual-purpose nickel (Ni) template synthesis. b, Scanningelectron microscopy (SEM) image of a graphene-encapsulated Simicroparticle (SiMP@Gr). The inset gives a broader view, showing many Simicroparticles encapsulated by the graphene cage. c, Transmissionelectron microscopy (TEM) image of an individual particle of SiMP@Gr. d,High resolution TEM image of the graphene cage's layered structure. Theintensity plot shows that 10 layers span a distance of about 3.34 nm(average inter-layer distance: about 0.334 nm), an indication ofgraphene layers. e, TEM image of the hollow graphene cage after etchingSi in NaOH. f, X-ray photoelectron spectroscopy (XPS) pattern of Si 2ppeaks of bare and graphene-encapsulated SiMP. The Si 2p peak isdramatically decreased with the graphene cage indicating a conformalcoating. g, Raman spectra of amorphous carbon-coated (SiMP@aC) andgraphene-encapsulated SiMP.

FIG. 3: Particle-level electrical and mechanical characterization ofgraphene cage by in situ TEM. a, Diagram of an electrical circuit forcurrent-voltage measurements and external load testing. b,Current-voltage data of graphene-encapsulated and amorphouscarbon-coated SiMP. Insets are live TEM images of their respectivecontact positions: graphene cage in upper left border and amorphouscarbon coating in low right border. The ohmic behavior shows that thegraphene cage's electrical resistance (about 17 kΩ) is a hundredfoldless than that of the amorphous carbon coating (about 1.4 MΩ). c,Schematic and time-lapse TEM images of external load testing on emptyshells of amorphous carbon. After a slight deformation, the brittleamorphous carbon shell cracks, destroying its spherical structure. d,The graphene cage exhibits good flexibility during an external load. Itsshape can be fully collapsed during compression and returns undamaged toits original structure after de-loading.

FIG. 4: In situ TEM observation of graphene cage Si lithiation. a,Diagram of the nanoscale electrochemical cell for in situ(de)lithiation. b, Time-lapse images of the lithiation ofgraphene-encapsulated SiMP. The Si microparticle (outlined) fracturesabruptly and violently within the mechanically strong graphene cage(outlined), which remains intact throughout the highly anisotropicprocess.

FIG. 5: Electrochemical characterization of graphene cage Si anodes.Specific capacities are reported based on the total mass of activematerials (Si and C). a, Reversible delithiation capacity ofgraphene-encapsulated SiMP with zero conductive additives. Bare andamorphous carbon-coated SiMP are control samples with carbon blackconductive additives. The mass loading of all active materials was about0.8 mg/cm². The rate was about C/20 for initial three cycles and aboutC/2 (about 1.5 mA/cm²) for later cycles (1 C=4.2 A/g Si). Theoreticalcapacity (370 mAh/g) of graphite electrode is shown in horizontal dashedline. Coulombic efficiency of the graphene-encapsulated SiMP is plottedon the secondary y-axis. b, c, Cross-sectional SEM images ofgraphene-encapsulated (b) and bare (c) SiMP electrodes before (left) andafter (right) cycling. d, Ex situ TEM image of graphene-encapsulatedSiMP after 3 cycles. White arrows indicate particle fracture is confinedwithin the graphene cage. Inset shows graphene cage (outlined) remainsfully intact. e, High-mass-loading cells (about 2.1 mg/cm²) with highareal capacity (about 5.2 mAh/cm²) of graphene-encapsulated SiMP cycledat about 0.2 mA/cm² for initial three cycles and about 1.0 mA/cm² forlater cycles. f, First-cycle voltage profiles of individual cells withcorresponding Coulombic efficiencies. For statistics on Coulombicefficiencies, see FIG. 12. g, Electrochemical impedance spectroscopy(EIS) measurements of graphene-encapsulated, bare, and amorphouscarbon-coated SiMP. Note that the graphene cage exhibits faster surfacekinetics than bare and amorphous carbon-coated SiMP with no observablechange even after 250 cycles.

FIG. 6: a, SEM image of bare SiMP. Note the highly non-uniformdistribution of size and shape. b, Size distribution statistics of bareSiMP.

FIG. 7: SEM image of Ni-coated SiMP. Inset is a higher magnification SEMimage showing conformal Ni coating despite highly non-uniform size andshape distribution of SiMP.

FIG. 8: a, Digital image of bare Si nanoparticles (left) andmicroparticles (right). Each vial contains about 0.4 g of particles.Note that on the microscale, Si has about 10 times more volumetriccapacity than on the nanoscale. b, Digital image ofgraphene-encapsulated SiMP. The powders were dark black, in contrast tothe starting gray color of the bare SiMP.

FIG. 9: a, TEM image of SiMP@Gr. b, Selected area diffraction of SiMP@Grshows crystalline structure of silicon. c, Dark field TEM shows locationwhere diffraction pattern from (b) originates.

FIG. 10: Thermogravimetric analysis (TGA) of bare andgraphene-encapsulated SiMP in about 80% argon and about 20% oxygen. Massloss is due to carbon oxidation of the graphene cage into gaseousspecies. Taking slight Si oxidation into account, the graphene cageaccounts for about 9% of the composite's weight.

FIG. 11: Voltage profiles of SiMP@Gr at various cycle numbers exhibittypical electrochemical features of Si. After 300 cycles, thegraphene-encapsulated SiMP still retains over about 85% of its chargecapacity. Under the same cycling conditions, bare and amorphouscarbon-coated microscale Si suffered severe capacity decay after 10 and30 cycles, respectively. For these control experiments, carbon blackconductive additives were added to improve electrical connectivitybetween particles. The capacity decay becomes even worse when theseconductive additives are removed.

FIG. 12: Statistics of first-cycle Coulombic efficiencies of bare,graphene-encapsulated, and amorphous carbon-coated SiMP. Value above barindicates average Coulombic efficiency of 10 cells and the error barsdenote the standard deviation. On average, the initial cycle Coulombicefficiency is improved by over about 10%.

FIG. 13: Deep galvanostatic cycling performance of 4 SiMP@Gr coin cellswith no rate change at about C/8 (C=4.2 Ah/g). Stable cycling behavioris achieved despite the absence of conductive additives.

FIG. 14: a, Galvanostatic cycling performance of SiMP@Gr at differentcurrent densities (C=4.2 Ah/g). Even without conductive additives, aspecific capacity of about 500 mAh/g is achieved at a rate of about 4 C.Impressively, this is comparable to Si nanoparticle rate performance. b,Voltage profiles at different current densities.

FIG. 15: a, Schematic of sodium hydroxide etching (about 2 M NaOH forabout 4 h) of SiMP@Gr. The self-supporting graphene cage remains intactafter etching. b, SEM image of empty graphene cage (scale bar: 5 μm).Inset shows stable morphology (scale bar: 1 μm). c, TEM image of emptygraphene cage (Scale bar: 1 μm). The strong and flexible frameworkmaintains its structural integrity despite no interior Si to support itsshape.

FIG. 16: Graphene cage encapsulating lithium iron phosphate (LFP)(cathode material).

FIG. 17: Schematic of a battery including a graphene-encapsulatedmaterial.

DETAILED DESCRIPTION

Nanostructuring has generated significant progress in addressing theproblems of high capacity Si anodes. However, Si nanomaterials are stillat high cost and not yet scalable due to complex synthesis processes,and issues with poor Coulombic efficiencies of nanostructured Si remain.The low Coulombic efficiencies of nanostructured Si can be caused by alarge surface area available to form a solid electrolyte interface(SEI), and, when coated with amorphous carbon, can be caused byirreversible trapping of Li by dangling bonds of the amorphous carboncoating. Micron-sized Si particles (or Si microparticles or SiMP) arelow-cost alternatives, but suffer from particle fracture duringelectrochemical cycling, leading to severe battery capacity decay.

Here, some embodiments are directed to encapsulating SiMPs usingconformally synthesized cages of multi-layered graphene, a material thatis mechanically strong, electrically conducting, and largely chemicallyinert. Advantageously, these desirable properties allow SiMP, for whichstable cycling was previously a challenge, to have excellent batteryperformance. The graphene cage acts as a strong and flexible bufferduring deep galvanostatic cycling, allowing the low-cost SiMP (e.g.,about 1-3 μm) to expand and fracture within the cage while retainingelectrical connectivity on both the particle and electrode level.Without the use of conductive additives, graphene-encapsulated SiMPexhibits the longest cycle life (e.g., at least about 85% capacityretention after 300 cycles) and highest areal capacity (e.g., at leastabout 5.2 mAh/cm²) reported for microscale Si. Furthermore, the graphenecage forms a chemically stable SEI, resulting in first-cycle Coulombicefficiencies as high as about 93.2% (or more), and about 99.9% (or more)within the first 10 cycles. By conferring favorable mechanical,electrical, and chemical properties to the composite, the conformalgrowth of the graphene cage demonstrates a strategy to overcome failuremodes in energy storage materials.

More generally, some embodiments are directed to direct growth ofconformal graphene cages or shells onto various materials. Graphene is amaterial with a wide range of desirable properties, including highmechanical strength and flexibility, electrical conductivity, andchemical stability. These properties can be successfully imparted tosilicon (a lithium-ion battery active electrode material) by theconformal growth of graphene cages. This allows a previously impracticalbattery material to have excellent cycle life and battery performance.The graphene encapsulation strategy disclosed herein is applicable toother materials that suffer from failure modes during device operation.More generally in materials synthesis, the graphene encapsulationstrategy can impart electrical conductivity, mechanical strength, andchemical stability to various materials. In energy applications, thegraphene encapsulation strategy can allow energy storage materials withunstable morphologies to perform well during operation. In someembodiments, a conformal nickel catalyst coating is used for direct,low-temperature graphene growth. Operations in synthesis can beimplemented by industrial processes that are readily scalable andreadily adaptable to various materials. The graphene encapsulationstrategy provides a graphene cage structure that simultaneously provideselectrical conductivity, mechanical strength, and chemical stability toa composite. Moreover, the versatile strategy results in a highlyconformal graphene cage and is successful even for particles withextremely non-uniform distributions of size and shape.

In some embodiments, a conformal graphene-encapsulated material includesparticles of a material, such as a battery electrode material likesilicon or other energy storage material, and hollow, encapsulatingstructures in the form of graphene cages, where one or more particlesare disposed within each graphene cage. The battery electrode material(or other encapsulated material) can include, for example, one or moreof carbon (C), graphite, silicon (Si), silicon monoxide (SiO), silicondioxide (SiO₂), tin (Sn), and tin oxides. The graphene cages can havesizes (e.g., outer lateral dimensions) in the range of about 10 nm toabout 100 μm, such as about 10 nm to about 200 nm, about 10 nm to about500 nm, about 100 nm to about 800 nm, about 200 nm to about 10 μm, about500 nm to about 3 μm, about 500 nm to about 10 μm, about 1 μm to about10 μm, or about 10 μm to about 100 μm. Walls of the graphene cages canhave a thickness in the range of about 0.5 nm to about 100 nm, such asabout 1 nm to about 90 nm, about 1 nm to about 70 nm, about 1 nm toabout 50 nm, about 1 nm to about 30 nm, about 1 nm to about 10 nm, orabout 1 nm to about 5 nm, and can be composed of a single graphene layeror two or more graphene layers, such as three or more, four or more,five or more, six or more, and so forth. The graphene cages can behighly graphitic, as characterized by, for example, a percentcrystallinity according to X-ray diffraction measurements of at leastabout 60%, at least about 70%, at least about 80%, at least about 90%,or at least about 95%, and up to about 97% or more, up to about 98% ormore, or up to about 99% or more. Also, the graphene cages can beelectrically conductive, as characterized by, for example, an electricalresistance of about 200 kΩ or less, about 150 kΩ or less, about 100 kΩor less, about 50 kΩ or less, or about 20 kΩ or less, and down to about17 kΩ or less, or down to about 15 kΩ or less. The battery electrodematerial (or other encapsulated material) is disposed within thegraphene cages, with a void or an empty space remaining within aninterior of each graphene cage. The battery electrode material can beprovided as solid particles, such as solid micron-sized particles ormicroparticles, but also can be provided as nano-sized particles,nanowires, nanotubes, hollow particles, or inner shells. For example,the battery electrode material can be provided as particles having sizes(e.g., outer lateral dimensions) in the range of about 10 nm to about100 μm, such as about 10 nm to about 200 nm, about 10 nm to about 500nm, about 100 nm to about 800 nm, about 200 nm to about 10 μm, about 500nm to about 3 μm, about 500 nm to about 10 μm, or about 1 μm to about 10μm.

In some embodiments, each graphene cage defines an internal volume, anda battery electrode material (or other encapsulated material) isdisposed within the internal volume and occupies about 100% of theinternal volume. In other embodiments, each graphene cage defines aninternal volume, and a battery electrode material (or other encapsulatedmaterial) is disposed within the internal volume and occupies less thanabout 100% of the internal volume, thereby leaving a void or an emptyspace inside the graphene cage to allow for expansion of the material.In some embodiments, such as for the case of a battery electrodematerial is its substantially delithiated state, a ratio of the volumeof the void inside the graphene cage relative to the volume of thebattery electrode material inside the graphene cage is in the range ofabout 1/20 to about 20/1, such as from about 1/10 to about 10/1, fromabout 1/10 to about 5/1, from about 1/10 to about 3/1, from about 1/10to about 2/1, from about 1/10 to about 1/1, from about 1/5 to about 3/1,from about 1/5 to about 2/1, from about 1/5 to about 1/1, from about 1/3to about 3/1, from about 1/3 to about 2/1, from about 1/3 to about 1/1,from about 1/2 to about 3/1, from about 1/2 to about 2/1, from about 1/2to about 1/1, from about 2/3 to about 3/1, from about 2/3 to about 2/1,or from about 2/3 to about 1/1. In some embodiments, such as for thecase of the battery electrode material is its substantially delithiatedstate, the volume of the void can be at least about 1/20 of the totalinternal volume inside the graphene cage, such as at least about 1/10,at least about 1/5, at least about 1/3, at least about 1/2, or at leastabout 2/3, with a remainder of the internal volume inside the graphenecage taken up by the battery electrode material. In some embodiments,each graphene cage is a monolithic or unitary encapsulating structure.The loading of the battery electrode material within the graphene cagescan be controlled so that there is sufficient active material whileensuring enough empty space for the material to expand duringlithiation. In some embodiments, a weight ratio of the battery electrodematerial relative to a combined mass of the battery electrode materialand the graphene cages is in the range of about 1% to about 99%, such asfrom about 10% to about 99%, from about 30% to about 99%, from about 50%to about 99%, from about 70% to about 99%, from about 80% to about 95%,or from about 85% to about 95%.

In some embodiments, an extent of conformal coverage of a batteryelectrode material (or other encapsulated material) by graphene cagescan be characterized according to X-ray photoelectron spectroscopy (XPS)or another surface spectroscopy technique. In the case of XPS, forexample, an initial scan can be performed (without sputtering) toevaluate atomic composition of surfaces of a graphene-encapsulatedmaterial to a depth of up to about 2 nm, and surface atomicconcentration ratios can be evaluated according to ratios ofcharacteristic peak intensities. In some embodiments, a surface atomicconcentration ratio of silicon (or other element included in the batteryelectrode material or other encapsulated material) relative to carbon(included in the graphene cages) can be about 1/10 or less, about 1/15or less, about 1/20 or less, about 1/25 or less, about 1/30 or less,about 1/35 or less, or about 1/40 or less, and down to about 1/45 orless, or down to about 1/50 or less.

In some embodiments, a method of forming a conformalgraphene-encapsulated material includes coating a material (to beencapsulated and serving as a substrate) with a metal catalyst to form ametal catalyst-coated material, growing graphene on the metalcatalyst-coated material to form a graphene cage encapsulating the metalcatalyst-coated material, and at least partially removing the metalcatalyst to form a void inside the graphene cage. The metal catalyst canbe, for example, nickel (Ni) or other suitable metal such as, forexample, iron (Fe), cobalt (Co), or copper (Cu), or a metal alloy, andcoating the material with the metal catalyst can be via, for example,electroless deposition or other deposition technique. Growing graphenecan be via, for example, carburization, such as by immersing, exposingor otherwise contacting the metal catalyst-coated material with acarbon-containing source, followed by annealing, such as at atemperature in the range of about 100° C. to about 900° C., about 200°C. to about 700° C., about 300° C. to about 550° C., or about 400° C. toabout 500° C., for about 10 min to about 5 h or about 0.5 h to about 3 hin an inert atmosphere. Growing graphene can be via, for example,decomposition of the carbon-containing source at elevated temperature.Removing the metal catalyst can be via, for example, an etchant, such asby immersing, exposing or otherwise contacting thegraphene-encapsulated, metal catalyst-coated material with a solutionincluding the etchant.

The conformal graphene-encapsulated materials described herein can beused for a variety of batteries and other electrochemical energy storagedevices. For example, the conformal graphene-encapsulated materials canbe included in electrodes for lithium-ion batteries or other types ofbatteries. As shown in an embodiment of FIG. 17, a resulting battery 100can include a cathode 102, an anode 104, and a separator 106 that isdisposed between the cathode 102 and the anode 104. The battery 100 alsocan include an electrolyte 108, which is disposed between the cathode102 and the anode 104. The anode 104 can include graphene-encapsulatedsilicon (or other graphene-encapsulated anode active material), and thecathode 102 can be a conventional cathode used in Li-ion batteries,Li—O₂ batteries, or Li—S batteries. The anode 104 can be substantiallydevoid of conductive additives; for example, the anode 104 can consistessentially of, or can consist of, a combination of thegraphene-encapsulated anode active material, a suitable binder, and acurrent collector. Graphene-encapsulated structures of otherelectrochemically active materials can be included within the cathode102 of some embodiments.

EXAMPLE

The following example describes specific aspects of some embodiments ofthis disclosure to illustrate and provide a description for those ofordinary skill in the art. The example should not be construed aslimiting this disclosure, as the example merely provides specificmethodology useful in understanding and practicing some embodiments ofthis disclosure.

This example sets forth an improved strategy to utilize micron-sized Siparticles (SiMPs with sizes of, for example, about 1-3 μm) as a low-costsource of battery electrode materials. The approach is to directly growa conformal graphene cage onto SiMP with some pre-designed empty spaceto confine particle fracture and mitigate against electrolyteinfiltration. Without a conformal coating, organic electrolyte wouldcontact and react with Si uncontrollably, resulting in an unstable SEI.With the graphene cage protecting SiMP, excellent battery performance isdemonstrated: stable cycling (e.g., at least about 85% capacityretention after 300 cycles), large areal capacity loading (e.g., atleast about 5.2 mAh/cm²), and high first-cycle (e.g., at least about93.2%) and later-cycle (e.g., at least about 99.9%) Coulombicefficiencies.

FIG. 1a shows the challenges of utilizing SiMPs. Si particles largerthan about 150 nm and Si nanowires larger than about 250 nm have beenshown to fracture during lithiation. During lithiation, SiMP would bebroken into small nano-sized Si particles, losing electrical contactwith each other and increasing overall surface area to form additionalSEI. To allow stable cycling of SiMPs, the conformal growth of aconductive graphene cage is introduced as a desirable encapsulationmaterial for improving SiMP battery performance (FIG. 1b ). The graphenecage structure imparts its desirable properties to Si, affording thefollowing advantages: (1) Despite particle fracture of SiMP, themechanically strong and flexible graphene cage with a pre-engineeredempty space remains undamaged and confines substantially all of thebroken Si pieces within the cage. When using multi-layered graphene, thegliding motion between individual graphene layers during SiMP volumeexpansion can facilitate the caging effect without breaking. (2) Theelectrical contact between fractured Si particles within each conductinggraphene cage is preserved. (3) The graphene cage's intrinsically highelectrical conductivity and ionic permeability through defects allowSiMPs to be electrochemically active. (4) The SEI is expected to formmainly on the graphene cage if there is no significant electrolyteleaking through the conformal cage. Despite the potential existence ofsome defects, the surface chemistry of the graphene cage is similar tothat of graphite, allowing stable SEI formation on graphite andresulting in high first and later-cycle Coulombic efficiency.

It is noted that the graphene cage approach is fundamentally differentfrom other approaches of graphene coatings. Instead of relying onphysical mixing or chemical vapor deposition of Si in graphene flakesfor an incomplete and non-conformal graphene coverage, the graphene cageapproach involves directly growing the graphene cage onto Si particlesin a conformal manner. Moreover, this versatile strategy is successfulfor Si microparticles with extremely non-uniform distributions of sizeand shape (See FIG. 6). This results in a highly conformal graphene cagewith a built-in and tunable void space. The graphene cage approach hereis also different from nanostructured Si carbon yolk-shell andpomegranate structures based on the following two aspects: (1) fractureprone micron-sized Si particles; (2) highly graphitic carbon to increaseSEI stability and Coulombic efficiency.

Synthesis and Characterization of Graphene Cage

The graphene cage encapsulation should be highly conformal in order forSi to inherit the remarkable properties of graphene. Internal emptyspace is also desirable for Si expansion and fracture within thegraphene cage. To this end, a synthesis approach is developed using adual-purpose Ni template. The Ni serves as both a metal catalyst forgraphene growth and the sacrificial layer for providing void space (FIG.2a ). Using electroless deposition, SiMPs are conformally coated withNi, the thickness of which can be tuned for the appropriate void space.Next, a benign carburization process activates the Ni-coated SiMP forlow-temperature (e.g., about 450° C.) graphene growth via adissolution-precipitation mechanism. Lastly, the Ni catalyst is etchedaway using FeCl₃ aqueous solution, opening up the void space for SiMPexpansion within the graphene cage (FIGS. 2b, c , and FIG. 9). Themulti-layered structure of the graphene cage (e.g., about 10 nm inthickness) is observed from the transmission electron microscopy (TEM)image (FIG. 2d ). Note that the cage exhibits a wavy structure due toconformal graphene growth along the large grains of Ni deposited ontothe SiMP (See FIG. 7). The mechanically robust graphene cage remainscontinuous throughout the curved regions, which act as a buffer toaccommodate acute interior stresses during particle fracture. Even aftercomplete removal of Si by NaOH aqueous solution, the self-supportinggraphene cage still remains structurally stable (FIG. 2e , and FIG. 15).Raman spectroscopy reveals the highly graphitic nature of the cage ascompared with amorphous carbon synthesized based on other approaches(FIG. 2g ). The pronounced D band with narrow bandwidth indicates thatsufficient defects are present to facilitate Li ion transport to Si.Furthermore, the considerably screened Si peak (FIG. 2f ) from X-rayphotoelectron spectroscopy (XPS) provides evidence for the conformalnature of the graphene cage, which makes up just about 9% of thecomposite's total mass (See FIG. 10). With less carbon content, thegraphene cage reduces the possibility of irreversibly trapping Li ionswithout compromising the specific capacity of the composite.

Electrical and Mechanical Behavior of Graphene Cage

To substantiate the impressive characteristics of the graphene cage, itselectrical and mechanical behavior is examined on the single particlelevel using a piezo-controlled, electrical biasing TEM-atomic forcemicroscopy (AFM) holder. A circuit was built by sandwiching thegraphene-encapsulated SiMP between a conducting gold (Au) substrate anda sharp tungsten (W) tip (FIG. 3a ). Monitoring the live TEM image (FIG.3b inset) ensured good electrical contact. By measuring current as afunction of applied voltage (FIG. 3b ), the electrical resistance of thegraphene cage is determined (about 17 kΩ) to be about hundredfold lessthan that of an amorphous carbon coating (about 1.4 MΩ) synthesizedaccording to other approaches. This is a remarkable result whenconsidering the low-temperature synthesis of the graphene cage (about450° C.) is actually lower than that of the amorphous carbon (about 800°C.). The electrically conductive graphene-encapsulated SiMP can allowelectrical connectivity through even thick electrodes constructed freeof any conductive additives.

With the same experimental configuration, the piezo-controllers are usedto apply an external load onto empty cages of graphene and amorphouscarbon to observe their mechanical deformation in situ. From FIG. 3c ,it is observed that the fragile amorphous carbon sphere cracks andbreaks after a slight deformation. This brittleness constrains itsability to contain microscale Si's violent particle fracture. Incontrast, the graphene cage exhibits resilience to an external load(FIG. 3d ) due to its mechanical strength and flexibility. The graphenecage is able to fully collapse its shape during compression, and itreturns to its original structure intact after the load is removed.Coupled with its superb electrical conductivity, these distinctqualities of the graphene cage make it well-suited to address silicon'schallenges during lithiation and delithiation.

In Situ Lithiation of Graphene-Encapsulated SiMP

The lithiation process of the graphene-encapsulated SiMP is revealed insitu by using a nanoscale electrochemical cell inside the TEM equipment(FIG. 4a ). These experiments allow direct observation of silicon'sintrinsic volume expansion and particle fracture during batteryoperation. From FIG. 4b , the SiMP appears to expand slowly until theparticle finally fractures in a vigorous fashion. Despite the abrupt andviolent rupture of the interior Si, the graphene cage preserves itsstructural integrity throughout the process, unlike amorphous carbon,which has been shown to readily crack. It is noted that the volumeexpansion appears highly anisotropic, indicative of crystallinesilicon's tendency to favor expansion in certain crystallographicdirections over others. This microscale anisotropic expansion is aprimary reason why other secondary coatings are impractical for SiMP.Non-uniform void space would have to be exactly engineered along thespecific crystallographic directions where expansion is favored;otherwise, the rigid and fragile coating would break. In contrast, thegraphene cage is shown to be mechanically strong and flexible. Thisallows it to survive the large interior stresses during microscalesilicon's anisotropic expansion and particle fracture, while stillretaining electrical contact between the fractured particles.

Electrochemical Performance

The graphene cage's mechanical strength and electrical conductivityallow SiMP, a previously impractical anode material, to have outstandingbattery performance. Specifically, the following can be achieved: (1)long cycling stability, (2) high areal/specific capacity, and (3) highinitial and later-cycle Coulombic efficiency without using anyconductive additives. From these data, the chemical stability of thegraphene cage is also apparent. Type 2032 coin cells were constructed(see methods) for deep galvanostatic cycling tests from about 1 to about0.01 V. Reported capacities are based on the total mass of Si and C inthe graphene cage composite.

As shown in FIG. 5a , the reversible capacity of thegraphene-encapsulated SiMP reached about 3300 mAh/g at a current densityof about C/20 (1 C=4.2 A/g). The high capacity indicates that the activematerials are electrically well-connected and fully participate inelectrochemical lithiation and delithiation (See FIG. 11). Furthermore,this is achieved without the use of any conductive additives, displayingthe excellent electrical conductivity of the graphene cage. From the4^(th) to 300^(th) cycle, continued cycling at a higher rate of aboutC/2 (about 1.5 mA/cm²) resulted in capacity retention of over about 85%for 300 cycles. After that, over about 1400 mAh/g of capacity remained,which is still about four times that of graphite's theoretical capacity.This cycling stability and rate performance is among the best reportedfor microscale Si to date and far surpasses that of bare or amorphouscarbon-coated Si (FIG. 5a ).

This cycling stability is attributed to the well-designed graphene cagestructure. From the ex-situ TEM images in FIG. 5d , it is observed thatthe graphene cage stays intact while the fractured microscale Si remainselectrically connected on the particle level. Additionally,cross-sectional scanning electron microscope (SEM) images show that thegraphene cage's built-in void space prevents large changes in electrodethickness (about 4.5% change in thickness, FIG. 5b ), allowing thefractured Si to maintain electrical contact on the electrode level.Without the stabilizing feature of the graphene cage, bare Simicroparticles would quickly become electrically disconnected from eachother, resulting in unstable particle morphology and catastrophicelectrode swelling by about 150% and eventual disintegration (FIG. 5c ).

In batteries, maintaining electrical conductivity and connectivity iseven more difficult at higher mass loadings because particles in thethicker electrode are increasingly distant from the current collector.Furthermore, small flaws on the particle level may propagate across thethicker electrode and cause significant capacity decay that is usuallyconcealed in lower mass-loading cells. The graphene-encapsulated SiMPalso addresses these electrode-level issues, exhibiting areal capacitiesof about 5.2 mAh/cm² (FIG. 5e ). At a mass loading of about 2.1 mg,these cells have specific capacity similar to the lower mass loadingcells in FIG. 5a (about 0.8 mg), an indication that substantially all ofthe graphene-encapsulated SiMP is still active at these higher massloadings. Achieving high-areal-capacity cycling without using anyconductive additives is further indication of the graphene cage'sintrinsic electrical conductivity and mechanical strength.

It is noted that in addition to cycling stability, high Coulombicefficiencies are desirable for the operation of a practical battery.Early cycle Coulombic efficiencies are especially important because theyare usually very low and account for most of the Li ion loss andelectrolyte consumption during SEI formation. As can be seen in typicalvoltage profiles (FIG. 5f ) and compiled coin cell statistics (See FIG.12), the initial-cycle Coulombic efficiency for bare SiMP is about 83%.This value drops considerably for an amorphous carbon coating (about74%) due to the high number of dangling carbon bonds acting as lithiumtrapping sites. Furthermore, even the best performing nanostructured Sianodes typically takes many cycles for the Coulombic efficiency to reachabove about 99%. In comparison, the graphene-encapsulated SiMP exhibitsinitial-cycle Coulombic efficiencies as high as about 93.2% (or more)(See FIG. 12) and quickly increases to about 99.5% (or more) within thefirst 5 cycles (FIG. 5a ). After 10 cycles, it reaches about 99.9% (ormore). For comparison, the first cycle Coulombic efficiency incommercial graphite anodes is about 90-94% and jumps to about 99.9% inthe early cycles.

The improvement in early and later-cycle Coulombic efficiency can beexplained in terms of two criteria: (1) surface chemistry should allowfor initial SEI formation without consuming too much Li, and (2)interface with electrolyte should be mechanically stable to preventadditional SEI formation. Commercial graphite anodes meet both of thesecriteria, allowing their tremendous Coulombic efficiency. Similarly, theelectrochemical cycling data of the graphene-encapsulated SiMP indicatethat the design also meets the criteria for high early and later-cycleCoulombic efficiency.

Firstly, the layered morphology (FIG. 2d ) of the conformal graphenecage is structurally and chemically similar to that of graphite, makingthe graphitized carbon atoms of the cage unlikely to trap Li.Furthermore, the use of micron-sized Si (See FIG. 8) lowers the surfacearea accessible by the electrolyte. Despite possible defects in thegraphene cage and the possibility of electrolyte leaking into some ofgraphene cages, the results show that the graphite-like surface and lowsurface area of the microscale Si allow the composite to achieve aninitial-cycle Coulombic efficiency approaching that of graphite. Inlater cycles, the mechanically stable graphene cage preventsuncontrolled SEI formation. This is supported by the Nyquist plotobtained from electrochemical impedance spectroscopy (EIS), where thesemicircle represents the charge-transfer resistance. The surfacekinetics of the graphene-encapsulated SiMP are much faster than that ofthe bare or amorphous carbon-coated SiMP, and this behavior also remainslargely unchanged even after 250 cycles (FIG. 5g ). Along with about99.9% Coulombic efficiency in later cycles (FIG. 5a ), the EIS dataprovide strong evidence for a stable SEI layer during cycling of thegraphene-encapsulated SiMP. These results indicate that thegraphene-encapsulated SiMP is a successful design for an activematerial.

Conclusion

This example sets forth an innovative synthesis approach to encapsulateSi with a graphene cage structure exhibiting a suite of desirableproperties. These properties give even microscale Si, a materialparticularly susceptible to particle fracture and unstable SEIs, greatbattery performance. Without the use of conductive additives, highcapacity cells are cycled (e.g., at least about 1400 mAh/g) over 300times with excellent initial-cycle (e.g., at least about 93.2%) andlater-cycle (e.g., at least about 99.9%) Coulombic efficiency at highcurrent densities (e.g., at least about 1.5 mA/cm²). In addition, thestrict design criteria demanded of stable, high-areal-capacity cells(e.g., about 5.2 mAh/g) further validate the graphene cage encapsulationstrategy. These values are among the best reported for Si microparticlesto date. This strategy can also be expanded to include a wide range ofother materials that fail in electrochemical reactions (See FIG. 16 foran example of a graphene cage encapsulating a lithium iron phosphatecathode material). By imparting mechanical strength, electricalconductivity, and chemical stability to the composite, encapsulation bya graphene cage demonstrates a powerful method to address failure modesin electrodes, making energy dense, low-cost battery materialsrealistic.

Methods

Synthesis. As depicted in FIG. 2a , a dual-purpose template approach isused to synthesize graphene-encapsulated SiMP with built-in void space.Surface-activated SiMPs were first placed in an electroless Ni bath atroom temperature for about 30 min. The thickness of the Ni coating canbe tuned to provide the appropriate void space. The Ni-coated SiMPpowder was then immersed in triethylene glycol (or other carbon source)for about 8 h at about 185° C. This carburization operation activatesthe Ni catalyst for graphene growth via a dissolution-precipitationmechanism at about 450° C. under argon (Ar) for about 1 h. Finally, theNi catalyst and native SiO₂ layer were sequentially etched in about 1.0M FeCl₃ and about 10% HF solution (or other etchant), respectively. Theamorphous carbon coating was synthesized by pyrolysis based on previouswork. For more details, see Supplementary Information.

In-situ TEM. A piezo-controlled, electrical biasing TEM-AFM holder(Nanofactory Instruments) was used to observe the (de)lithiation processof graphene-encapsulated SiMP and measure the graphene cage's electricaland mechanical properties. Li metal and SiMP@Gr were dispersed ontoabout 0.25 mm W and copper (Cu) wires, respectively, and were thenbrought into contact by the piezo-controller. By applying a voltage biasof about −3 V, Li ions flowed through the Li metal's nativeoxide/nitride to alloy with Si at the working electrode. The graphenecage remained intact despite the violent anisotropic fracture of theSiMP. To determine the graphene cage's current-voltage behavior, SiMP@Grwas dropcast onto an about 0.25 mm Au wire. A bare W wire with a sharptip was used to contact the graphene cage, completing the circuit.Measuring current as a function of applied voltage confirms that thegraphene cage is over 2 orders of magnitude more electrically conductivethan amorphous carbon. This obviates the need for conductive additivesin electrochemical cells, and it also affords great rate performance(See FIG. 14). For external load testing, the piezo-controller was usedto push the W tip into the graphene cage. The reversible deformationmakes it a suitable encapsulation material for anisotropic SiMPexpansion and fracture.

Electrochemistry. SiMP@Gr working electrodes for cycling stability (FIG.5a , FIG. 13), rate capability (FIG. 14), and high areal capacity testswere prepared using a conventional slurry method. SiMP@Gr powders andpolyvinylidene fluoride (PVDF) binder with a mass ratio of about 9:1were dispersed in N-methyl-2-pyrrolidone (NMP) in the absence of anyconductive additives and stirred for about 12 h. Control electrodes withbare SiMP or amorphous carbon-coated SiMP (SiMP@aC) were prepared usingthe same slurry method, except with about 8:1:1 mass ratio of activematerial, carbon black conductive additive, and PVDF binder. Aftercasting onto an about 15 μm thick Cu foil and drying at about 50° C. ina vacuum oven, the samples were calendared and cut into about 1 cm²circular disks with a mass loading of about 0.8 mg/cm². These workingelectrodes were then assembled into type 2032 coin cells with Li metalas the counter/reference electrode. The electrolyte used was about 1.0 MLiPF₆ in about 89 vol. % 1:1 w/w ethylene carbonate/diethyl carbonatewith about 10 vol. % fluoroethylene carbonate and about 1 vol. %vinylene carbonate. All coin cells were evaluated by galvanostaticcycling between about 0.01 and about 1 V versus Li/Li⁺. The specificcapacity for all cells was calculated using the total mass of thegraphene-encapsulated SiMP composite. Charge/discharge rates werecalculated assuming silicon's theoretical capacity (4200 mAh/g).Coulombic efficiency was calculated using the ratio of delithiation(C_(dealloy)) capacity to lithiation (C_(alloy)) capacity(C_(dealloy)/C_(alloy)×100%).

Supplementary Information

Activating SiMP for Electroless Ni Deposition

Silicon's surface is densely coated with a nucleation seed (palladium(Pd) in this case) for a conformal Ni coating. Polydopamine (about 3 nm)is used as a surface-adherent layer to sensitize the Si surface withSn(II) ions, which will subsequently reduce the Pd metal seed fromsolution onto Si.

In a typical synthesis, about 2 g SiMP (about 1-3 μm; US ResearchNanomaterials, Inc.) were dispersed in about 160 mL of deionized (DI)water and sonicated for about 10 min. About 1.6 mL of Tris-buffer (about1.0 M; pH of about 8.5; Teknova) and about 320 mg dopamine hydrochloride(Sigma-Aldrich) were sequentially added to the aqueous solution andstirred at room temperature for about 1 h. This will form a very thinlayer of polydopamine that will help the Ni-nucleation seed adhere morereadily to the silicon surface. Next, about 5 mL of stannous chlorideaqueous solution (about 5 g/L SnCl₂; about 10 mL/L hydrochloric acid(HCl); Sigma-Aldrich) is directly added to the mixture and stirred foran additional about 1 h. The decrease in pH prevents the polydopaminelayer from growing thicker. The sample is then collected bycentrifugation and washed three times with DI water. Finally, theparticles are immersed in about 15 mL of palladium chloride aqueoussolution (about 0.5 g/L PdCl₂; about 6.25 mL/L HCl; Sigma-Aldrich) andstirred for about 1 h. Washing 3 times with DI water and collecting bycentrifugation results in activated SiMP.

Electroless Ni Deposition

The thickness of the Ni coating can be tuned by either changing theconcentration of the electroless Ni (EN) solution or controlling thenumber of deposition reactions. In this case, a combination of both wasused. Two electroless Ni solutions were prepared: a primary solution(about 20 g/L nickel sulfate hexahydrate; about 10 g/L sodium citratedihydrate; about 5 g/L lactic acid) and a secondary solution with doublethe component concentration (about 40 g/L nickel sulfate hexahydrate;about 20 g/L sodium citrate dihydrate; about 10 g/L lactic acid).Activated SiMP will be sequentially immersed in these EN solutions.

Prior to the first electroless deposition, about 1 g of dimethylamineborane (DMAB; Sigma-Aldrich) and about 2 mL of ammonium hydroxide(NH₃.H₂O, Sigma-Aldrich, about 28%) were added to about 180 mL of theprimary EN solution. The pH-sensitive DMAB serves as the reducing agentduring electroless Ni deposition. About 500 mg of activated SiMP arethen added to the dilute EN solution and gently stirred for about 30min. Bubbles will begin to effervesce and the green-colored EN solutionshould appear lighter in color. After deposition is complete, theSiMP@1×Ni will settle to the bottom. While holding the SiMP@1×Niparticles at the bottom of the container with a magnet, the depleted ENbath is carefully poured out. In a separate container, about 2 g of DMABand about 3 mL of ammonium hydroxide are added to about 180 mL of thesecondary EN solution. This is then added immediately to the dampparticles (SiMP@1×Ni) and stirred for about 30 min. The resultingSiMP@2×Ni is washed twice with ethanol and dried in a vacuum oven atabout 50° C. for about 1 h.

Carburization, Annealing, and Etching of Dual-Purpose Ni Template

Dried SiMP@2×Ni (about 2.3 g after EN) is dispersed in about 150 mL oftriethylene glycol (Santa Cruz Biotechnology, Inc.) and about 500 μL ofabout 50% w/w aqueous NaOH solution. After stirring at about 185° C. forabout 8 h, the carburized SiMP@2×Ni was collected by centrifugation andwashed 3 times with ethanol. The carburization process occurs when theorganic solvent decomposes, allowing carbon atoms to diffuse into the Nilayer and adhere to the surface. This primes the SiMP@2×Ni forlow-temperature graphene growth. Samples were then dried in a vacuumoven at about 50° C. for about 1 h. The dried particles were placed in atube furnace with the following temperature profile: heat to about 100°C. at about 2° C./min; heat to about 450° C. at about 20° C./min; holdtemperature at about 450° C. for about 1 h. An Ar flow rate of about 80sccm was maintained throughout the annealing process. The dual-purposeNi template and native oxide layer on Si were etched by sequentiallyimmersing the annealed particles in about 1 M FeCl₃ (about 2 h) andabout 10 vol. % hydrogen fluoride aqueous solution (about 30 min),respectively. Graphene-encapsulated SiMP were obtained (about 400 mg)after washing 3 times with ethanol and drying in a vacuum oven at about50° C. for about 1 h (FIG. 8).

Synthesis of Amorphous Carbon-Coated SiMP

About 500 mg of SiMP were dispersed in about 120 mL water. About 4 mL ofCetrimonium bromide (CTAB, Sigma-Aldrich, 10 mM) and about 0.4 mLammonia were added and the solution was stirred for about 20 min toensure the adsorption of CTAB on the silicon surface. Next, about 100 mgresorcinol (Sigma-Aldrich) and about 140 μL formaldehyde solution(Sigma-Aldrich, about 37% wt % in H₂O) were added and stirred overnight.The final coated Si was collected by centrifugation and washed withethanol three times. The coating was carbonized under Ar at about 800°C. for about 2 h.

Materials Characterization

The weight percentage of Si and C in the graphene-encapsulated SiMP wasdetermined from the weight loss curves measured under simulated airatmosphere (about 20% O₂+about 80% Ar, both are ultra-purity grade gasesfrom Airgas) on a thermogravimetric (TG)/differential thermal analysis(DTA) instrument (Netzsch STA 449) with a heating rate of about 5°C./min. Under these conditions, mass increases due to slight Sioxidation, whereas carbon oxidation to gaseous species causes mass loss.To decouple these two processes, a bare Si control sample was measuredat the same heating conditions (See FIG. 10) and the mass gain wassubtracted from the graphene-encapsulated SiMP curve. The Si content inthe composite is then the lowest point of the corrected curve (about91%), leaving about 9% from the carbon in the graphene cage. Othercharacterization was carried out using SEM (FEI Sirion, Nova NanoSEM),TEM (FEI Tecnai, Titan), XPS (SSI S-Probe Monochromatized, Al Kαradiation at 1486 eV), EIS (BioLogic VMP3), and Raman spectroscopy(Horiba JY).

Electrochemical Characterization

Working electrodes were prepared using a conventional slurry method.SiMP@Gr powders and PVDF (Kynar HSV 900) binder with a mass ratio ofabout 9:1 were dispersed in NMP in the absence of any conductiveadditives and stirred for about 12 h. Control electrodes with bare SiMPor SiMP@aC were prepared using the same slurry method, except using amass ratio of about 8:1:1 for active material, carbon black conductiveadditive (Super P, TIMCAL, Switzerland), and PVDF binder, respectively.After casting onto an about 15 μm thick Cu foil and drying at about 50°C. in a vacuum oven for about 3 h, the samples were calendared and cutinto about 1 cm² circular disks with a mass loading of about 0.8 mg/cm².In an Ar-filled glovebox, these working electrodes were assembled intotype 2032 coin cells with a polymer separator (Celgard 2250) and Limetal (Alfa Aesar) as the counter/reference electrode. About 100 μL ofabout 1.0 M LiPF6 in about 89 vol. % 1:1 w/w ethylene carbonate/diethylcarbonate (BASF Selectilyte LP40) with about 10 vol. % fluoroethylenecarbonate and about 1 vol. % vinylene carbonate (Novolyte Technologies)was added as the electrolyte with full wetting of both working andcounter electrode surfaces.

Coin cells were loaded into a battery tester (Arbin Instruments) andcycled between about 0.01 and about 1 V versus Li/Li⁺. The specificcapacity for all cells was calculated using the total mass of thegraphene-encapsulated SiMP composite. Charge/discharge rates werecalculated assuming silicon's theoretical capacity (4200 mAh/g Si).Coulombic efficiency was calculated using the ratio of delithiation(C_(dealloy)) capacity to lithiation (C_(alloy)) capacity(C_(dealloy)/C_(alloy)×100%).

For ex-situ SEM/TEM characterization of working electrodes, coin cellswere charged to about 1 V and disassembled. The working electrodes werethen rinsed gently in acetonitrile to remove Li salts from the residualelectrolyte.

As used herein, the singular terms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to an object can include multiple objects unless thecontext clearly dictates otherwise.

As used herein, the term “set” refers to a collection of one or moreobjects. Thus, for example, a set of objects can include a single objector multiple objects. Objects of a set can be the same or different.

As used herein, the terms “substantially,” “substantial,” and “about”are used to describe and account for small variations. When used inconjunction with an event or circumstance, the terms can refer toinstances in which the event or circumstance occurs precisely as well asinstances in which the event or circumstance occurs to a closeapproximation. For example, when used in conjunction with a numericalvalue, the terms can refer to a range of variation of less than or equalto ±10% of that numerical value, such as less than or equal to ±5%, lessthan or equal to ±4%, less than or equal to ±3%, less than or equal to±2%, less than or equal to ±1%, less than or equal to ±0.5%, less thanor equal to ±0.1%, or less than or equal to ±0.05%.

As used herein, the terms “connect,” “connected,” “connecting,” and“connection” refer to an operational coupling or linking. Connectedobjects can be directly coupled to one another or can be indirectlycoupled to one another, such as through another set of objects.

As used herein, the term “size” refers to a characteristic dimension ofan object. Thus, for example, a size of an object that is spherical canrefer to a diameter of the object. In the case of an object that isnon-spherical, a size of the non-spherical object can refer to adiameter of a corresponding spherical object, where the correspondingspherical object exhibits or has a particular set of derivable ormeasurable characteristics that are substantially the same as those ofthe non-spherical object. When referring to a set of objects as having aparticular size, it is contemplated that the objects can have adistribution of sizes around the particular size. Thus, as used herein,a size of a set of objects can refer to a typical size of a distributionof sizes, such as an average size, a median size, or a peak size.

Additionally, amounts, ratios, and other numerical values are sometimespresented herein in a range format. It is to be understood that suchrange format is used for convenience and brevity and should beunderstood flexibly to include numerical values explicitly specified aslimits of a range, but also to include all individual numerical valuesor sub-ranges encompassed within that range as if each numerical valueand sub-range is explicitly specified. For example, a ratio in the rangeof about 1 to about 200 should be understood to include the explicitlyrecited limits of about 1 and about 200, but also to include individualratios such as about 2, about 3, and about 4, and sub-ranges such asabout 10 to about 50, about 20 to about 100, and so forth.

While the disclosure has been described with reference to the specificembodiments thereof, it should be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the true spirit and scope of the disclosure asdefined by the appended claims. In addition, many modifications may bemade to adapt a particular situation, material, composition of matter,method, operation or operations, to the objective, spirit and scope ofthe disclosure. All such modifications are intended to be within thescope of the claims appended hereto. In particular, while certainmethods may have been described with reference to particular operationsperformed in a particular order, it will be understood that theseoperations may be combined, sub-divided, or re-ordered to form anequivalent method without departing from the teachings of thedisclosure. Accordingly, unless specifically indicated herein, the orderand grouping of the operations is not a limitation of the disclosure.

What is claimed is:
 1. A method of forming a conformalgraphene-encapsulated material, comprising: coating a battery electrodematerial with a metal catalyst to form a metal catalyst-coated batteryelectrode material, wherein coating the battery electrode material withthe metal catalyst includes performing electroless deposition of themetal catalyst on the battery electrode material; growing graphene onthe metal catalyst-coated battery electrode material to form a graphenecage encapsulating the metal catalyst-coated battery electrode material;and at least partially removing the metal catalyst to form a void insidethe graphene cage.
 2. The method of claim 1, wherein the metal catalystis nickel.
 3. The method of claim 1, wherein growing the grapheneincludes exposing the metal catalyst-coated battery electrode materialto a carbon-containing source, followed by performing carburization andannealing.
 4. The method of claim 3, wherein annealing is performed at atemperature in the range of 200° C. to 600° C.
 5. The method of claim 3,wherein exposing the metal catalyst-coated battery electrode material tothe carbon-containing source includes disposing the metalcatalyst-coated battery electrode material in a solution of thecarbon-containing source.
 6. The method of claim 1, wherein removing themetal catalyst is through the graphene cage.
 7. The method of claim 1,wherein removing the metal catalyst is via an etchant.
 8. The method ofclaim 1, wherein the battery electrode material includes silicon.
 9. Themethod of claim 1, wherein the battery electrode material is provided asat least one particle having a dimension in the range of 200 nm to 10μm.